The present invention relates to a process for the continuous digestion of cellulosic fiber material.
Through U.S. Pat. No. 3,802,956 it is known that wood can be impregnated with white liquor in countercurrent flow and that black liquor can be added to the wood material at the inlet to the impregnation vessel. The object of this procedure is primarily to increase the concentration of active chemicals in the digesting liquor by withdrawing a certain amount of impregnation liquid in which the content of active chemicals has been substantially consumed. The liquid to wood ratio in the digester is thereby lowered, thus giving a high concentration of active chemicals which results in rapid digestion. The smaller amount of liquid in the digester in comparison with conventional methods also results in less steam consumption, particularly high-pressure steam. However, it has been found that a low liquid to wood ratio may entail problems in controlling the cooking process, as well as difficulties with the movement of the chip column due to differences in relative speed between chips and free liquid. The relatively high concentration of chemicals in alkaline digestion processes also causes attack on the carbohydrates in the raw wood, resulting in lowered pulp viscosity and pulp strength.